1. Field of the Invention
This invention relates generally to the recovery of alcohols from concentrated aqueous acid streams.
2. Description of the Prior Art
Large volumes of alcohols are produced annually by the catalytic hydration of olefins, in which the selected olefin feed is absorbed in a concentrated aqueous sulfuric acid stream to form the corresponding alcohol and alkyl ester of the sulfuric acid. For example, the absorption of butene to form sec-butanol and a sec-butyl ester of sulfuric acid can be illustrated by the following equation: ##STR1## Thereafter, water is admixed with the sulfuric acid stream withdrawn from the absorber to hydrolyze the ester and to facilitate alcohol recovery which is accomplished by stripping with steam. There is thereby produced a diluted sulfuric acid stream which must for economic reasons be treated to concentrate it with respect to its sulfuric acid content, after which it is recycled to the olefin absorption step.
The reconcentration of the diluted sulfuric acid stream is a very expensive and energy-intensive process step, and a method whereby the alcohol could be recovered from the sulfuric acid stream withdrawn from the absorber, which did not require such a reconcentration, would be highly desirable.
Processes have been proposed for alcohol recovery by extraction from such alcohol-containing sulfuric acid streams by use of benzene, chloroform, ether, carbon bisulfide and toluene (U.S. Pat. No. 1,365,046); phenols, cresols, their homologues, ethers and phosphates (U.S. Pat. No. 2,139,953); and saturated hydrocarbons (British Pat. No. 493,884). British Pat. No. 506,473 relates to a method for production of organic oxy-compounds from a mixture of two liquid phases containing the oxy-compound distributed between the two layers in which the layers are separated and then recontacted at a lower temperature to extract the oxy-compound from the more hydrophobic phase, e.g., a hydrocarbon phase.
However, such extraction methods are not commercially desirable, since the solvents extract very little alcohol if the acid strength of the alcohol-containing sulfuric acid stream is greater than about 55%. Dilution of the acid stream to improve the ability of these solvents to extract the alcohol is not economically practicable, since even more expense would be incurred in reconcentrating the resulting sulfuric acid (recovered after the extraction) prior to its being recycled to the olefin absorbing step.
C. L. Munson et al., Ind. Eng. Chem. Proc. Des. Dev., vol. 23, no. 1, pp. 109-115 (1984) (which is not admitted herein to be prior art) investigated equilibrium distribution coefficients and separation factors for extraction of ethanol from dilute aqueous solutions of the alcohol by a number of different solvents and solvent mixtures, including extraction solvents comprising neodecanoic acid and 2-ethyl hexanoic acid. An ethanol-water-extraction solvent phase is obtained and treated to dehydrate the ethanol, followed by fractionating the ethanol and solvent mixture. The dilute aqueous raffinate is separated from the extractor and treated, as by stripping, to separate extraction solvent dissolved in the raffinate.
U.S. Pat. No. 3,489,779 to Esso Research and Engineering Company is directed to a process for purifying neo-carboxylic acids by treating the neo acid with concentrated sulfuric acid (85+ wt. % H.sub.2 SO.sub.4) followed by treatment with a phase-separating agent, which can either comprise water or a paraffinic hydrocarbon. The function of the concentrated sulfuric acid is to convert olefinic and other impurities (e.g., Fe) into the corresponding sulfates. If water is added as the phase-separating agent it must be used in carefully controlled amounts, sufficient to liberate the neo-acid from its sulfuric acid complex but insufficient to hydrolyze the alkyl sulfates and other impurities formed in the first step. A water and neo-acid mixture, containing a lessened impurity level, is recovered. When a paraffinic hydrocarbon is used, a paraffin and neo-acid mixture is recovered as a separate liquid phase, and the sulfate impurities remain in the sulfuric acid phase. In the latter process alternative, the separated paraffin/neo-acid layer still contains 5-10 wt. % H.sub.2 SO.sub.4 which must be removed.